Fix renamer panic
This patch fixes Trac #12216 and #12127. The 'combine' function in 'imp_occ_env' in RnNames.filterImports checked for an empty field-selector list, which was (a) unnecessary and (b) wrong. I've elaborated the comments. This does NOT fix #11959 which is related but not the same (it concerns bundling of pattern synonyms).
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